Light emitting diodes (LEDs) are typically constructed by growing a p-n diode on a substrate. The diode is constructed by growing a layer of n-doped material on the substrate, growing a light generation region on the n-doped layer, and then growing the layer of p-doped material on top of the n-doped material. An electrode is then deposited on the top surface of the p-doped layer, and a contact is made to the n-doped layer. Light may be extracted either through the substrate or through the top surface of the p-doped material or through the top electrode. If the light is to be removed through the top electrode, the electrode is constructed from a transparent material such as indium tin oxide or thin gold film.
The efficiency of an LED is the product of two efficiencies, the efficiency with which power applied to the electrodes is converted to light and the efficiency with which that light is coupled out of the device. For a normal LED fabricated on a substrate, a large fraction of the light generated in the diode is lost because of poor coupling efficiency. Most semiconductors have an index of refraction that is much higher than that of air or the epoxy encapsulants. Accordingly, only light impinging on the surface of the diode in a small cone of angles will escape the surface due to total internal reflection. Most of the remaining light is reflected back into the diode layers and is trapped in a waveguide bounded by the substrate surface and the top surface of the diode. Much of this trapped light is eventually absorbed within the device. Accordingly, the efficiency of semiconductor diode is less than ideal.
One method that has been suggested for improving the extraction efficiency of an LED requires the LED to be macroscopically shaped such that light generated in the device strikes the surface at the critical angle or smaller, thereby preventing the total internal reflection. The problem can be avoided if the chip is shaped as a hemisphere or truncated pyramid. Such shaping of the chip is very cumbersome and quite costly.
A second prior art method is disclosed in U.S. Pat. No. 6,812,161, the contents are incorporated herein by reference. Here, a method for improving the extraction efficiency of a GaAs-based LED utilizes a roughening of the upper surface of the LED by etching to destroy the planar nature of the surface thereby providing a large variety of non-planar facets through which light striking the surface can exit.
The prior methods for roughening the surface involve a random etching of the top surface of the LED. For example, an irregular etch pattern can be generated by depositing particles on the surface of the LED and then using the particles to define a random etch mask. For these methods of the prior art, two processes were used to fabricate the roughened surface: first, diameters of the particles were reduced by selective etching, and second, surface structures were fabricated by dry etching using the particles as masks. The sidewall of the resulting surface structures created by the prior methods are nearly vertical as the selected etching process for creating the surface structures does not etch the particles, which served purely as masks. These cylinder-like structures (as shown in FIG. 3) may still limit extraction efficiency.
The internal quantum efficiency of LED is almost 100% due to the development of growth technologies. However, the light extraction efficiency for a normal LED is very low due to total internal reflection at the interface between the semiconductor and air. Typically, for a normal GaAs based LED, only ˜2% light generated in active layer can be extracted. This problem has partly been solved by surface roughing or surface texturing. However, low cost and high throughput technologies for surface texturing are desired. U.S. Pat. No. 6,812,161 discloses a method to fabricate LEDs with randomly arranged pillar structures by using spherical particles as masks. In addition, almost surface textures formed on LED are simple post-like structures as shown in FIG. 3 or hole arrays (random or ordered). Such simple structures limit further increase of light extraction efficiency which strongly depends on the material system, emitting wavelength, layer structures and surface structures of LEDs.